Bose-Einstein
condensates can sometimes explode like a pocket-sized supernova,
but when they're not doing that they can look like the inside
of a cigar bar, awash in insubstantial smoke rings. These
two disparate images of one of modern physics's most mysterious
creations come out of recent research efforts at the Commerce
Department's National
Institute of Standards and Technology in cooperation
with the University of Colorado at Boulder. Details of the
work, which include some quite unexpected observations,
were announced for the first time today at the American
Physical Society March Meeting in Seattle, Wash.
Bose-Einstein
condensation of atomic gases was predicted in theory by
Albert Einstein in 1924, and was first observed by NIST
and University of Colorado scientists in 1995 at their joint
institute, JILA,
located on the CU-Boulder campus. When the condensate is
produced, virtually all the atoms in the gas fall into the
lowest-energy quantum mechanical state. Spread out in space,
they become superimposed on one another, each indistinguishable
from the other, creating what has been called a "superatom."
A collection of millions of atoms, extending over the entire
gas, behaves like a single atom described by the "matter
wave" picture of quantum mechanics.
A legitimately
"new state of matter," Bose-Einstein condensates
exhibit a variety of odd, large-scale quantum effects. Techniques
in the laser-cooling of atoms pioneered at NIST were an
important step in achieving the extraordinarily cold temperatures-only
a few billionths of a degree above absolute zero-needed
to produce a BEC, and have opened many novel possibilities
for manipulating and studying of the behavior of quantum
wave functions. BECs can exhibit the interference phenomenon
of wave motion, for example, in which the crests and troughs
of two waves cancel each other. They can also produce "matter
wave lasers," which emit bright beams of atoms with
identical speeds.
Building
Your Own Supernova
Working
at JILA, physicist Carl Wieman's [pronounced wy-man] team
has explored tuning the self-interaction of atoms in a BEC.
By making a BEC in a particular isotope of rubidium, rubidium-85,
and then changing the magnetic field in which the BEC is
sitting, the team is able to adjust the wavefunction's self-interaction
between repulsion and attraction. If the self-interaction
is repulsive, all the parts of the wavefunction push each
other away. If it is attractive, they all pull towards each
other, like gravity. Achieving a pure BEC in rubidium-85
required the cloud of atoms to be cooled to about 3 billionths
of a degree above absolute zero, the lowest temperature
ever achieved.
Making
the self-interaction mildly repulsive causes the condensate
to swell up in a controlled manner, as predicted by theory.
However, when the magnetic field is adjusted to make the
interaction attractive, dramatic and very unexpected effects
are observed.
The
condensate first shrinks as expected, but rather than gradually
clumping together in a mass, there is instead a sudden explosion
of atoms outward. This "explosion," which actually
corresponds to a tiny amount of energy by normal standards,
continues for a few thousandths of a second. Left behind
is a small cold remnant condensate surrounded by the expanding
gas of the explosion. About half the original atoms in the
condensate seem to have vanished in that they are not seen
in either the remnant or the expanding gas cloud.
Since
the phenomenon looks very much like a tiny supernova, or
exploding star, Wieman's team dubbed it a "Bosenova."
The most surprising thing about the Bosenova is that the
fundamental physical process behind the explosion is still
a mystery.
"Understand
that atoms have been very well studied. Essentially all
the behavior of isolated atoms in general and BECs in particular
we thought were quite well understood, and could be predicted
accurately by theoretical calculations," said Wieman.
"Even for those features that cannot be accurately
predicted, the basic physical processes are still qualitatively
well understood.
"But
the theoretical calculations of what would happen in this
situation predict behaviors that are totally unlike what
we've observed, so the basic process responsible for the
Bosenova must be something new and different from what has
been proposed," Weiman said.
Wieman's
team has made a detailed study of the Bosenova, including
how it progresses in time, how it depends on the strength
of the self-interaction of the condensate, and many other
parameters. The fate of the missing atoms is still an open
question, but the researchers suspect that they wind up
either accelerated so greatly that they escape the trap
undetected, or perhaps form molecules that are invisible
to the detection system.
Solitons
to "Smoke Rings"
In
related work, experimental and theoretical teams under Eric
Cornell of both NIST and CU-Boulder, and Charles Clark of
NIST, have for the first time demonstrated vortex rings
created in a Bose-Einstein condensate. Such vortices have
been conjectured to occur in superfluids such as liquid
helium or a BEC, but have never been observed directly.
A superfluid-typically liquid helium when it is within about
2 degrees of absolute zero-is one of the stranger manifestations
of quantum theory on a macroscopic scale. Flowing without
viscosity or friction, like current in a superconductor,
a superfluid will flow uphill despite gravity, or up and
over the walls of a container.
A vortex
is a whirling pattern of liquid, a circular flow around
a central core (the most common examples are tornados and
whirlpools.) The core can be made to close upon itself,
making a "vortex ring" - a familiar example is
the smoke ring.
Certain experimental results led theorists to suspect that
closed vortex rings occur in superfluids and play a crucial
role in their behavior, but in a dense superfluid like liquid
helium the rings are difficult to observe. A BEC, on the
other hand, is an ultra-low density gas, but it exhibits
the properties of a superfluid. The creation of permanent
vortex motions is one of the striking features of superfluids
and superconductors, and in 1999 the NIST/CU team at JILA
first produced a simple vortex structure in a Bose-Einstein
condensate. Computer simulations by the theoretical team
predicted that vortex rings should occur in a BEC and be
large enough to be seen.
To
blow smoke rings in a BEC, the research team first created
a strange phenomenon called a "dark soliton" in
a spherical BEC of rubidium-87. A soliton is a special kind
of wave that can travel for a long distance, maintaining
its shape and velocity without dissipating its energy. The
ripples caused by throwing a pebble in a pond are ordinary
waves. A tsunami is a soliton. A dark soliton requires some
imagination-it's a soliton wave made up of the absence of
something.
Using
a combination of microwave radiation and a specially-tuned
laser beam played rapidly back and forth across the BEC,
the researchers split the rubidium atoms into two different
quantum states, creating a dark soliton in the center of
the atoms of one state, filled with atoms in the second
state. In photos the soliton is visible as a dark plane
bisecting the glowing sphere of the BEC. "Filling"
the soliton with other atoms is a special trick to give
stability to what is otherwise a very unstable phenomenon.
At
the proper moment a tuned laser beam is used to blow the
stabilizing atoms out of the soliton. At almost the same
moment, the trapping field that confines the BEC is turned
off. The suddenly unstable soliton collapses into more stable
vortex structures, and since the BEC is a sphere, it is
encouraged to decay into vortex rings. Without its filling,
the soliton and its decay products would still be too small
to observe, but with the trap turned off the condensate
rapidly expands in all directions, enlarging its internal
structures like the patterns on an inflating balloon. In
computer simulations the soliton can clearly be seen to
decay into a series of concentric vortex rings, exactly
like a particularly complicated set of smoke rings. The
experimental photos strongly resemble the simulation images.
The
teams plan further studies of the stability, lifetime and
dynamics of vortex rings in BECs.
Details
of these projects will be found in the following forthcoming
papers:
As a
non-regulatory agency of the U.S. Department of Commerce's
Technology Administration, NIST strengthens the U.S. economy
and improves the quality of life by working with industry
to develop and apply technology, measurements and standards
through four partnerships: the NIST Laboratories, the Baldrige
National Quality Program, the Manufacturing Extension Partnership
and the Advanced Technology Program.
JILA
is a joint research institute of the National Institute
of Standards and Technology and the University of Colorado.
Located on the main university campus in Boulder, Colo.,
the Institute is a center for teaching and research in the
areas of atomic, chemical, optical, laser, gravitational,
and solar physics; semiconductors; precision measurement;
astrophysics; and astronomy.
Editor's
note: Digital movies of these two NIST-JILA experiments
are available on the web:
Implosion
and explosion of a Bose-Einstein condensate "bosenova":
www.nist.gov/public_affairs/bosenova.htm
Decay
of a dark soliton into vortex rings in a Bose-Einstein condensate:
www.nist.gov/public_affairs/smokerings.htm
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